1. Field of the Invention
The present invention relates to a lens system for optically recorded disks.
2. Description of the Prior Art
With a lens system for optically recorded disks, the spot of light formed through it should exist on the track, which is formed by pits representing the signals recorded on the disk, always during operation and, moreover, the spot of light should be always kept in the best condition. On the other hand, the disk is rotated at high speed and generally causes vertical vibration, horizontal vibration, irregular rotation, etc.
As one of methods for making the spot follow up these high speed movements, there is such method that the lens system for optically recorded disks is moved upward and downward for the purpose of focusing by following up the vertical vibration of the disk and the light is vibrated by vibrating a galvanomirror or the like in order to follow up the movements at a right angle to the rotational direction of the disk such as horizontal vibration of the disk.
For the lens system for optically recorded disks to be used in the above-mentioned method to vibrate a galvanomirror, aberrations of the lens system should be favourably corrected to a certain field angle unlike the case of the method in which the lens systme for optically recorded disks is vibrated in order to follow up the movements.
Besides, the lens system for optically recorded disks should have a long working distance so that the lens system for optically recorded disks will not collide against the disk to cause damage when misoperation occurs.
Furthermore, to as far as possible reduce the loss in intensity of light to be caused by the lens system for optically recorded disks, the number of lenses constituting the lens system for optically recorded disks should be made as small as possible. Besides, surfaces of lenses which are in contact with air should be provided with anti-reflection coatings which have transmission factor increasing effect over the range of wavelengths of light to be used.
In these days, players for optically recorded disks are being put to practical use. Therefore, for lens systems for optically recorded disks, it is now important to fulfill the above-mentioned requirements and, especially, to make the working distance long.
When actually assembling the lens system for optically recorded disks into player for optically recorded disks, the lens system may be sometimes mounted in inclined state due to the error in mounting accuracy or the like. Therefore, in these days when the lens system for optically recorded disks are being put to practical use, the lens system for optically recorded disks should have a sufficiently large field angle.
Moreover, the semiconductor laser which produces oscillated light with long wavelength is also being put to practical use in these days. Therefore, compared with those days when He-Ne laser was used, a lens system for optically recorded disks with still larger N.A. is required at present.
Furthermore, for practical use, it is desirable that the price of the lens system is low.
As it is necessary to fulfill many requirements as described in the above, the lens system for optically recorded disks should have a small outer diameter, large N.A., wide field angle, long working distance, and favourably corrected chromatic aberration over the range of wavelengths of light to be used.
However, among known lens systems for optically recorded disks, there is almost no lens system which fulfills all of the above-mentioned requirements. For example, in case of the lens system disclosed in Japanese published unexamined Patent Application No. 127339/79, the number of lenses consituting the lens system is small, i.e., four lenses. However, working distance is short, N.A. is small and field angle is narrow. The lens system disclosed in Japanese published unexamined Patent Application No. 26545/80 comprises four lenses and has a long working distance. However, its N.A. is small and field angle is narrow. In case of the lens system disclosed in Japanese published unexamined Patent Application No. 134815/80, it is attempted to eliminate chromatic aberration by adopting four-component five-element lens configuration. However, chromatic aberration is not eliminated satisfactorily. Moreover, its working distance is short, N.A. is small and field angle is narrow.